In the present paper, the author discusses Bion's concept of group mentality, and the methods used by the group to express it in order to counteract individual deviance from the dominant basic assumption as discussed by Bion. Group mentality is defined here as "the unanimous expression of the will of the group, contributed to by the individual in ways of which he is unaware, influencing him disagreeably whenever he thinks or behaves in a manner at variance with the basic assumptions.". After introducing some basic Bionian concepts, the author describes some of the most frequently observed expressions of group mentality. That is, he adumbrated six expressions, namely, rejection by prohibition, the "pliers" method, ignorance, negative echoeing, myth creation, and silence. These forms of group mentality expressions are described and illustrated by means of clinical vignettes extracted from D-groups (diagnostic groups) lead by the author. In his famous Experiences in Groups, Bion has attracted our attention to the existence of group mentality, without providing us with information about its forms of expression. Therefore, the purpose of the present study is to hed more light on this concept, and its elationship with other Bionian concepts, namely group culture and basic ssumption group.